BALDWIN I. (d. 1205), emperor of Romania, count of Flanders and Hainaut, was one of the most prominent leaders of the fourth crusade, which resulted in the capture of Constantinople, the conquest of the greater part of the East Roman empire, and the foundation of the Latin empire of Romania. The imperial crown was offered to, and refused by, Henry Dandolo, doge of Venice. The choice then lay between Baldwin and Boniface of Montferrat. Baldwin was elected (9th of May 1204), and crowned a week later. He was young, gallant, pious and virtuous, one of the few who interpreted and observed his crusading vows strictly; the most popular leader in the host. The empire of Romania was organized on feudal principles; the emperor was feudal superior of the princes who received portions of the conquered territory. His own special portion consisted of Constantinople, the adjacent regions both on the European and the Asiatic side, along with some outlying districts, and several islands including Lemnos, Lesbos, Chios and Tenos. The territories had still to be conquered; and first of all it was necessary to break the resistance of the Greeks in Thrace and secure Thessalonica. In this enterprise (summer of 1204) Baldwin came into collision with Boniface of Montferrat, the rival candidate for the empire, who was to receive a large territory in Macedonia with the title of king of Saloniki. He hoped to make himself quite independent of the empire, to do no homage for his kingdom, and he opposed Baldwin's proposal to march to Thessalonica. The antagonism between Flemings and Lombards aggravated the quarrel. Baldwin insisted on going to Thessalonica; Boniface laid siege to Hadrianople, where Baldwin had established a governor; civil war seemed inevitable. An agreement was effected by the efforts of Dandolo and the count of Blois. Boniface received Thessalonica as a fief from the emperor, and was appointed commander of the forces which were to march to the conquest of Greece.

During the following winter (1204-1205) the Franks prosecuted conquests in Bithynia, in which Henry, Baldwin's brother, took part. But in February the Greeks revolted in Thrace, relying on the assistance of John (Kaloyan), king of Bulgaria, whose overtures of alliance had been unwisely rejected by the emperor. The garrison of Hadrianople was expelled. Baldwin along with Dandolo, the count of Blois, and Marshal Villehardouin, the historian, marched to besiege that city. The Bulgarian king led to its relief an army which far outnumbered that of the crusaders. The Frank knights fought desperately, but were utterly defeated (14th of April 1205); the count of Blois was slain, and the emperor captured. For some time his fate was uncertain, and in the meanwhile Henry, his brother, assumed the regency. Not till the middle of July was it definitely ascertained that he was dead. It seems that he was at first treated well as a valuable hostage, but was sacrificed by the Bulgarian monarch in a sudden outburst of rage, perhaps in consequence of the revolt of Philippopolis, which passed into the hands of the Franks. One contemporary writer says that his hands and feet were cut off, and he was thrown into a valley where he died on the third day; but the manner of his death is obscure. King John himself wrote to Pope Innocent III. that he died in prison. His brother Henry was crowned emperor in August.